A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is a key component (memory element) in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic read head. A TMR sensor typically has a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic insulator layer. The sensor stack in a so-called bottom spin valve configuration which is preferred for biasing reasons is generally comprised of a seed (buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer, tunnel barrier layer, free layer, and capping layer that are sequentially formed on a substrate. The free layer serves as a sensing layer that responds to external fields (media field) while the pinned layer is relatively fixed and functions as a reference layer. The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield which also serve as electrodes as described in U.S. Pat. No. 5,715,121. When a sense current is passed from the top shield to the bottom shield in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. A CPP transducer is disclosed in U.S. Pat. No. 5,668,688. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
A giant magnetoresistive (GMR) head is another type of memory device. In this design, the insulator layer between the pinned layer and free layer in the TMR stack is replaced by a non-magnetic conductive spacer such as copper.
In the TMR stack, the pinned layer may have a synthetic anti-ferromagnetic (SyAF) configuration in which an outer pinned layer is magnetically coupled through a coupling layer to an inner pinned layer that contacts the tunnel barrier. The outer pinned layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer which is magnetized in the same direction. The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
A TMR sensor is currently the most promising candidate for replacing a GMR sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantage of a TMR sensor is that a substantially higher MR ratio can be realized than for a GMR sensor. In addition to a high MR ratio, a high performance TMR sensor requires a low areal resistance RA (area×resistance) value, a free layer with low magnetostriction (λ) on the order of 1×10−8 to <5×10−6 and low coercivity (Hc) below 10 Oe such as CoFe/NiFe, a strong pinned layer, and low interlayer coupling (Hin) through the barrier layer. The MR ratio (also referred to as TMR ratio) is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um2.
MgO based TMR technology has been used primarily for high density magnetic recording due to its intrinsic high TMR ratio originating from so-called coherent tunneling which is directly related to wave function symmetry of the neighboring CoFe and MgO band structure. Currently, very large TMR values exceeding 100% can be obtained rather easily at RT with an RA of less than 10 ohm-μm2 which makes this technology easily adaptable to device applications, especially for read head sensors.
As recording density becomes higher and higher, it is critical to make the MgO tunnel barrier as thin as possible in order to match the resistance of the MTJ to other electronic components. Unfortunately, the TMR ratio drops drastically as the MgO barrier thickness becomes too thin. Also, the interlayer coupling (Hin) between the AP1 pinned layer and the free layer soars to a very high value as MgO thickness is thinned, making it vary difficult to adjust the bias point and causing performance degradation. Therefore, alternative sensors are needed that are more easily implemented in an ultra-low RA regime.
In the prior art, K. Belashchenko et al. in Phys. Review B 72, 140404 R (2005) have correlated the sharp TMR reduction at small MgO barrier thickness to the emergence of an interfacial resonant state controlled by the minority spin band. They predicted that a Ag monolayer epitaxially deposited at the interface between a CoFe pinned layer and MgO tunnel barrier would suppress the tunneling through the minority band and enhance the TMR ratio for thin MgO layers. Unfortunately, Ag is highly diffusive and is typically extremely difficult to grow well on CoFe surfaces. Thus, Ag insertion has not been successful in significantly improving TMR. Other approaches are necessary to reach a high TMR value with ultra-low RA.
F. Greullet et al. in “Large inverse magnetoresistance in fully epitaxial Fe/Fe3O4/MgO/Co magnetic tunnel junctions”, Appl. Phys. Left. 92, 053508 (2008) report that a large negative TMR value can be changed to a positive value by varying the applied bias. In a related paper by F. Greullet et al. entitled “Evidence of a Symmetry-Dependent Metallic Barrier in Fully Epitaxial MgO Based Magnetic Tunnel Junctions”, Phys. Rev. Left. Nov. 2; 99 (18) 187202 (2007), tunneling across an ultrathin Cr spacer inserted at the interface of a Fe/MgO/Fe(001) junction is described.
S. Yuasa et al. have reported the effect of inserting a thin non-magnetic copper layer between an aluminum oxide tunnel barrier and Co(001) ferromagnetic layer in “Spin-Polarized Resonant Tunneling in Magnetic Tunnel Junctions”, Science, Vol. 297, 234-237 (2002).
An ultra thin Cr spacer is inserted in a Fe/MgO/Fe(001) junction to promote quantum well states in an adjacent Fe layer as described by F. Greullet et al. in “Evidence of a Symmetry-Dependent Metallic Barrier in Fully Epitaxial MgO Based Magnetic Tunnel Junctions”, Phys. Rev. Lett. Vol. 18, 187202 (2007).
Resonant tunneling is also suggested for a configuration where a barrier is sandwiched between two quantum wells as described by J. Mathon and A. Umerski in “Theory of resonant tunneling in an epitaxial Fe/Au/MgO/Au/Fe(001) junction”, Phys. Rev. B, Vol. 71, 220402(R) (2005).
A. Vedyayev et al. in “Resonant spin-dependent tunneling in spin-valve junctions in the presence of paramagnetic impurities”, Phys. Rev. B, Vol. 63, 064429 (2001) describe the effect of impurities in the insulator in a Co/Al2O3/Co junction.
U.S. Pat. No. 7,333,306, U.S. Patent Appl. 2006/0034022, and U.S. Patent Appl. 2007/0188945 all refer to a spacer comprised of a confining current path (CCP) configuration in which a MgO layer with Cu paths formed therein is sandwiched between two Cu layers in a MTJ stack.
U.S. Patent Appl. 2007/0091514, U.S. Patent Appl. 2007/0242395, U.S. Patent Appl. 2008/0032159, and U.S. Patent Appl. 2007/0176519 all relate to a spin valve spacer made of Cu or MgO.
In U.S. Pat. No. 7,301,733, a nano oxide layer is inserted in a free layer or in a pinned layer but is kept a certain distance from a spacer layer in a spin valve stack.
U.S. Patent Appl. 2007/0164265 describes a dual spin valve containing both a Cu spacer and a MgO tunnel barrier but the two layers are separated by a free layer